Irregular Revving? Newish Merc 20HP

[Tonus]

Hi all - I took the boat out for the first time since last Sept '22. This is the third year I've owned my Mercury 20HP EFI 4-stroke, bought new from Excel in 2020. I went really easy on the engine, v steady at first and then cruising at about 5-6 knots for about an hour. After about 15mins I noticed that the revving became really irregular - revving downwards from where it was and then in one instance revving up higher and throwing me about a bit.

I wondered if that was normal for a 'first run' of a new season? A couple of other things to know:
I used some petrol from last year.
I've been running the engine every month or so with a hose connected for 5-10 mins but this was the first time the engine has been in gear and had throttle for 6 months.

I've attached a link to a video - it was worse than this half an hour before... engine noticeably shaking a bit when the revs dip. I kept the throttle still throughout.

Thanks

https://www.youtube.com/watch?v=0LbpiZl1OKs

[Steve509926]

Hi Tonus, did you have the vent open enough on the fuel tank? Possible kink in the fuel line from the tank to the OB?

[Tonus]

Ah yes I see how that could be an issue… I did have the vent open - perhaps not enough. I’ll try again next week and make sure it’s wide open.

The revving was really quite erratic at one point and I stopped the boat dead - like I said seemed to rev up suddenly. Wasn’t do bad after that but yeh it could be a blockage somehow.

I just thought it might be a common problem that people get with cold engines/dodgy petrol etc